Process to reduce plasma induced damage

ABSTRACT

Embodiments described herein provide thin film transistors (TFTs) and processes to reduce plasma induced damage in TFTs. In one embodiment, a buffer layer is disposed over a substrate and a semiconductor layer is disposed over the buffer layer. A gate dielectric layer is disposed over the semiconductor layer. The gate dielectric layer contacts the semiconductor layer at an interface. The gate electrode 204 is disposed over the gate dielectric layer. The gate dielectric layer has a Dit of about 5e10 cm−2eV−1 to about 5e11 cm−2eV−1 and a hysteresis of about 0.10 V to about 0.30 V improve performance capability of the TFT while having a breakdown field between about 6 MV/cm and about 10 MV/cm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. provisional patent applicationSer. No. 62/571,731, filed Oct. 12, 2017, which is herein incorporatedby reference.

BACKGROUND Field

Embodiments of the present disclosure generally relate to thin filmtransistors (TFTs) and a process to reduce plasma induced damage inTFTs.

Description of the Related Art

Flat panel displays are commonly used for active matrix displays such ascomputer and television monitors. Plasma-enhanced chemical vapordeposition (PECVD) is generally employed to deposit thin films on asubstrate, such as a transparent substrate for flat panel displayimplementations. PECVD is generally accomplished by introducing aprecursor gas or gas mixture into a vacuum chamber that contains asubstrate. The precursor gas or gas mixture is typically directed towardthe substrate through a distribution plate situated near a top of thechamber opposite the substrate. The precursor gas or gas mixture in thechamber is energized (e.g., excited) into a plasma by applying radiofrequency (RF) power to the chamber from one or more RF sources coupledto the chamber. The excited gas or gas mixture reacts to form a layer ofmaterial on a surface of the substrate.

Flat panels processed by PECVD techniques are typically large, oftenexceeding several square meters. Gas distribution plates (or gasdiffuser plates) utilized to provide uniform process gas flow over flatpanels are relatively large in size, particularly as compared to gasdistribution plates utilized for 200 mm and 300 mm semiconductor waferprocessing. Further, as the substrates are rectangular, edges of thesubstrate, such as sides and corners thereof, experience conditions thatmay be different than the conditions experienced at other portions ofthe substrate. These different conditions affect processing parameterssuch as film thickness, deposition uniformity, and/or film stress.

PECVD is oftentimes used to deposit films for TFTs. By the very natureof the PECVD process, plasma is formed. The plasma environment can beharsh and, oftentimes, cause damage. The damage may even occur to theunderlying layer upon which a subsequent layer is deposited.

Therefore, what is needed is for TFTs with reduced plasma damage and animproved process to reduce plasma induced damage.

SUMMARY

In one embodiment, a thin film transistor (TFT) is provided. The TFTincludes a substrate, a gate electrode disposed on the substrate, a gatedielectric layer disposed on the gate electrode and the substrate, asemiconductor layer disposed on the gate dielectric layer, and a sourceelectrode and a drain electrode disposed on the semiconductor layer. Thegate dielectric layer has a breakdown field between about 6 MV/cm andabout 10 MV/cm, an interface trap density (DO of about 5e¹⁰ cm⁻²eV⁻¹ toabout 5e¹¹ cm⁻²eV⁻¹, and a hysteresis of about 0.10 V to about 0.30 V.

In one embodiment, a thin film transistor (TFT) is provided. The TFTincludes a substrate, a buffer layer disposed on the substrate, asemiconductor layer disposed on the buffer layer, a gate dielectriclayer disposed on the semiconductor layer, a gate electrode disposed onthe gate dielectric layer, an inter-layer dielectric (ILD) layerdisposed on the gate dielectric layer and the gate electrode, and asource electrode disposed in a source electrode via of the ILD layer anda drain electrode is disposed in a drain electrode via of the ILD layer.The gate dielectric layer has a breakdown field between 6 MV/cm andabout 10 MV/cm, an interface trap density (D_(it)) of about 5e¹⁰ cm⁻²eV⁻¹ to about 5e¹¹ cm⁻² eV⁻¹, and a hysteresis of about 0.10 V to about0.30 V.

In one embodiment, a method of fabricating a thin film transistor (TFT)is provided. The method includes flowing a deposition gas at adeposition gas flow rate into a process volume of a chamber, applying aradio frequency (RF) power to the deposition gas for an initial intervalat an initial power level forming an initial zone a range of zones of agate dielectric layer, the initial zone having a zone density with aminimum density, and increasing the initial power level in intervalsforming zones of the range of zones until the RF power is applied for afinal interval at a final power level forming a final zone of the rangeof zones, the final zone having the zone density with a maximum density,and the zone density of each zone formed has a density not less than thezone density of a prior zone.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentdisclosure can be understood in detail, a more particular description ofthe disclosure, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlyexemplary embodiments and are therefore not to be considered limiting ofits scope, and may admit to other equally effective embodiments.

FIG. 1 illustrates a schematic, cross-sectional view of a PECVD chamberaccording to embodiments described herein.

FIG. 2 illustrates a schematic, cross-sectional view of a top gate TFTaccording to one embodiment.

FIG. 3A illustrates a schematic, cross-sectional view of an exemplarygate dielectric layer according to one embodiment.

FIG. 3B illustrates a schematic, cross-sectional view of an exemplarygate dielectric layer according to one embodiment.

FIG. 4 is a flow diagram of a method of fabricating a top gate TFTaccording to one embodiment.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements and features of oneembodiment may be beneficially incorporated in other embodiments withoutfurther recitation.

DETAILED DESCRIPTION

Embodiments described herein relate to TFTs with reduced plasma damageand processes to reduce plasma damage on a substrate or an alreadydeposited film during TFT fabrication. Plasma from the PECVD process candamage an already deposited film. Specifically, the exposed surface uponwhich a layer is to be deposited by the PECVD process can be damaged. Asdiscussed herein, a process is disclosed that reduces and/or eliminatesplasma damage.

Embodiments herein are illustratively described below in reference to aPECVD system configured to process large area substrates, such as aPECVD system, available from AKT, a division of Applied Materials, Inc.,Santa Clara, Calif. It is contemplated that other suitably configuredapparatus from other manufacturers may also be implemented according tothe embodiments described herein. In addition, it should be understoodthat various implementations described herein have utility in othersystem configurations, such as etch systems, other chemical vapordeposition systems, or other systems in which distributing gas within aprocess chamber is desired, including those systems configured toprocess round substrates.

FIG. 1 illustrates a schematic, cross-sectional view of a PECVD chamber100 for forming electronic devices for flat panel displays, such as thinfilm transistor (TFT) devices and active matrix organic light emittingdiode (AMOLED) devices. The chamber 100 includes walls 102, a bottom104, and a diffuser 110 which define a process volume 106. Morespecifically, the process volume 106 is further defined by surfaces 107of the walls 102. In one embodiment, the walls 102, bottom 104, anddiffuser 110 are fabricated from a metallic material, such as aluminum,stainless steel, and alloys thereof. For example, the diffuser 110 maybe formed from a 6061 aluminum alloy. In another embodiment, thediffuser 110 may be formed from an anodized aluminum material. Asubstrate support 130 is disposed in the process volume 106 opposite thediffuser 110. The process volume 106 is accessed through a sealable slitvalve opening 108 formed through the walls 102 such that a substrate 105may be transferred in and out of the chamber 100.

The substrate support 130 includes a substrate receiving surface 132 forsupporting a substrate 105 and a stem 134 coupled to a lift system 136to raise and lower the substrate support 130. In operation, a shadowframe 133 may be positioned over a periphery of the substrate 105 duringprocessing. Lift pins 138 are moveably disposed through the substratesupport 130 to move the substrate 105 to and from the substratereceiving surface 132 to facilitate substrate transfer. The substratesupport 130 may also include heating and/or cooling elements 139 tomaintain the substrate support 130 and substrate 105 positioned thereonat a desired temperature. The substrate support 130 may also includegrounding straps 131 to provide RF grounding at a periphery of thesubstrate support 130.

The diffuser 110 is coupled to a backing plate 112 adjacent a peripheryof the diffuser 110 by a suspension element 114. The diffuser 110 mayalso be coupled to the backing plate 112 by one or more center supports116 to help prevent sag and/or control the straightness/curvature of thediffuser 110. A gas source 120 is fluidly coupled to the backing plate112 to provide gas through the backing plate 112 to a plurality of gaspassages 111 formed in the diffuser 110 and ultimately to the substratereceiving surface 132.

A vacuum pump 109 is coupled to the chamber 100 to control the pressurewithin the process volume 106. An RF power source 122 is coupled to thebacking plate 112 and/or to the diffuser 110 to provide RF power to thediffuser 110 to generate an electric field between the diffuser 110 andthe substrate support 130. In operation, gases present between thediffuser 110 and the substrate support 130 are energized by the RFelectric field into a plasma. Various RF frequencies may be used, suchas a frequency between about 0.3 MHz and about 200 MHz. In oneembodiment, the RF power source 122 provides power to the diffuser 110at a frequency of 13.56 MHz.

A remote plasma source 124 is also coupled between the gas source 120and the backing plate 112. The remote plasma source 124 may be aninductively coupled remote plasma source, a capacitively coupled remoteplasma source, or a microwave remote plasma source, depending upon thedesired implementation. The remote plasma source 124 may be utilized toassist in process gas plasma generation and/or cleaning gas plasmageneration.

In one embodiment, the heating and/or cooling elements 139 embedded inthe substrate support 130 are utilized to maintain the temperature ofthe substrate support 130 and substrate 105 thereon during depositionbetween about 200 degrees Celsius to about 500 degrees Celsius or less.

Spacing between a top surface of the substrate 105 disposed on thesubstrate receiving surface 132 and a bottom surface 140 of the diffuser110 during deposition processes may be between 400 mil and about 1,200mil, for example between 400 mil and about 800 mil. The chamber 100 maybe used to deposit various materials, such as, silicon nitride material,silicon oxide material, amorphous silicon materials, for a variety ofapplications, including interlayer dielectric films and gate insulatorfilms, among others.

FIG. 2 illustrates a schematic, cross-sectional view of a top gate TFT200 according to one embodiment. The top gate TFT 200 includes asubstrate 202, a buffer layer 204, a semiconductor layer 206, a gatedielectric layer 208, a gate electrode 210, an inter-layer dielectric(ILD) layer 212, a source electrode 214, and a drain electrode 216. Thesubstrate 202 may comprise any suitable material such as silicon basedsubstrates, semiconductor based substrates, insulating based substrates,germanium based substrates, and, in general, one or more generic layersthat would be present in a CMOS structure. It is to be understood thatother materials are contemplated as well.

The buffer layer 204 is disposed over the substrate 202 and thesemiconductor layer 206 is disposed over the buffer layer 204. In oneembodiment, the buffer layer 204 includes at least one of a siliconmononitride (SiN) and a silicon dioxide (SiO₂) containing material, orthe SiO₂ containing material. The semiconductor layer 206 includes asemiconductor material. In one embodiment, the semiconductor materialincludes at least one of a silicon, a polysilicon, a low temperaturepolysilicon, an amorphous silicon, an indium-gallium-zinc oxide (IGZO),and an zinc oxynitride (ZnON) containing material. It is to beunderstood that other materials are contemplated as well. The gatedielectric layer 208 is disposed over the semiconductor layer 206. Inone embodiment, the gate dielectric layer 208 has a thickness of about500 Angstroms to about 1500 Angstroms. The gate dielectric layer 208contacts the semiconductor layer at an interface 201. The gatedielectric layer 208 includes an insulating material. In one embodiment,the insulating material includes at least one of a silicon mononitride(SiN), a silicon nitride (Si₃N₄), a silicon monoxide (SiO), a silicondioxide (SiO₂), and a silicon oxynitride (Si₂N₂O) containing material.It is to be understood that other materials are contemplated as well.The semiconductor layer 206 and the gate dielectric layer 208 may bedeposited by a plasma-enhanced chemical vapor deposition (PECVD)process.

The gate electrode 210 is disposed over the gate dielectric layer 208.ILD layer 212 is disposed over the gate electrode 210. A sourceelectrode 214 is disposed in a source electrode via 218 of the ILD layer212 and a drain electrode 216 is disposed in a drain electrode via 220of the ILD layer 212. The gate electrode 210, source electrode 214, anddrain electrode 216 each include a conductive material such as copper,titanium, tantalum, or any electrically conductive metal. It is to beunderstood that other materials are contemplated as well. In oneembodiment, the material used for gate electrode 210, source electrode214, and drain electrode 216 is the same material. In anotherembodiment, the material for at least one of the gate electrode 210,source electrode 214, and drain electrode 216 is different from thematerial used for the remaining two electrodes. In yet anotherembodiment, the material for the gate electrode 210, source electrode214, and drain electrode 216 is different for each electrode. The gateelectrode 210, source electrode 214, and drain electrode 216 may bedeposited by physical vapor deposition (PVD). The ILD layer 212 may bemade of any suitable dielectric material, such as silicon oxide. Thebuffer layer 204 and ILD layer 212 may be deposited by a PECVD process.

The gate dielectric layer 208 has a breakdown field between about 6megavolts per centimeter (MV/cm) and about 10 MV/cm. The breakdown fieldis a result of high ion bombardment, generally due to high (RF powerapplied to deposition gas, in the PECVD process to densify gatedielectric layer 208. In the PECVD process the interface 201 and thesemiconductor layer 206 may be damaged due to the high ion bombardmentof the PECVD process. Conventionally, due to the damage to interface 210and the semiconductor layer 206 from the high ion bombardment of thePECVD process, the gate dielectric layer 208 has an interface trapdensity (D_(it)) greater than about 5e¹¹ cm⁻² eV⁻¹ and a hysteresisgreater than about 0.30 V. A D_(it) greater than about 5e¹¹ cm⁻² eV⁻¹and hysteresis greater than about 0.30 V may reduce to performancecapability of the top gate TFT 200.

FIG. 3A is a schematic, cross-sectional view of an exemplary gatedielectric layer 208A. The exemplary gate dielectric layer 208A has aD_(it) of about 5e¹⁰ cm⁻²eV⁻¹ to about 5e¹¹ cm⁻² eV⁻¹ and a hysteresisof about 0.10 V to about 0.30 V improve performance capability of a thetop gate TFT 200 while having a breakdown field between about 6 MV/cmand about 10 MV/cm. The D_(it) of about 5e¹⁰ cm⁻²eV⁻¹ to about 5e¹¹cm⁻²eV⁻¹ and a hysteresis of about 0.10 V to about 0.30 V are a resultof a density profile through a range of zones of a thickness of the gatedielectric layer 208A.

The gate dielectric layer 208A has a first surface 304, a second surface306, and a thickness 308. The thickness 308 of the gate dielectric layer208A is divided into a range of zones 310 measured from the firstsurface 304 corresponding to 0% of the thickness 308 to the secondsurface 306 corresponding to 100% of the thickness 308. Each zone 312has a zone thickness and zone density. In one embodiment, each zone 312has a zone thickness of about 0.001% to about 20% of the thickness 308.A density profile through the range of zones 310 of the thickness 308has a minimum density and a maximum density. In one embodiment, aninitial zone 314 of the range of zones 310 adjacent to the first surface304 has a zone density with the minimum density, the final zone 316 ofthe range of zones 310 adjacent to the second surface 306 has a zonedensity with the maximum density, and the zone density each zone 312disposed immediately over a prior zone is not less than the zone densityof the prior zone. In one embodiment, the maximum density is about 2.1g/cm³ and the maximum density is about 2.25 g/cm³.

In one embodiment, each zone 312 has a zone thickness that issubstantially the same and each zone 312 disposed immediately over aprior zone has a zone density not less than the zone density of theprior zone. In another embodiment, the increase in the zone density is aresult of a PECVD process. The PECVD process includes applying a radiofrequency (RF) power to the deposition gas for an initial interval at aninitial power level forming the initial zone 314 a range of zones 310,and increasing the initial power level in intervals forming zones 312 ofthe range of zones 310 until the RF power is applied for a finalinterval at a final power level forming a final zone 316 of the range ofzones. Each zone 312 of the range of zones 310 has substantially thesame thickness and each zone 312 disposed immediately over a prior zonehas a zone density not less than the zone density of the prior zone.Referring to FIG. 2, the first surface 304 of the gate dielectric layer208A is in contact with the semiconductor layer 206 at the interface201. The gate dielectric layer 208A has the D_(it) of about 5e¹⁰ cm⁻²eV⁻¹ to about 5e¹¹ cm⁻² eV⁻¹ and the hysteresis of about 0.10 V to about0.30 V.

FIG. 3B is a schematic, cross-sectional view of an exemplary gatedielectric layer 208B. The exemplary gate dielectric layer 208B has aD_(it) of about 5e¹⁰ cm⁻² eV⁻¹ to about 5e¹¹ cm⁻² eV⁻¹ and a hysteresisof about 0.10 V to about 0.30 V improve performance capability of thetop gate TFT 200 while having a breakdown field between about 6 MV/cmand about 10 MV/cm. The D_(it) of about 5e¹⁰ cm⁻²eV⁻¹ to about 5e¹¹cm⁻²eV⁻¹ and a hysteresis of about 0.10 V to about 0.30 V are a resultof an intermediate layer of the gate dielectric layer 208B having athickness that is not greater than about 20% of the thickness of thegate dielectric layer 208B and the intermediate layer having a densitynot greater than the density of a bulk layer the gate dielectric layer208B.

The gate dielectric layer 208B has a first surface 304, a second surface306, an intermediate layer 301, a bulk layer 302, and a thickness 308.In one embodiment, the intermediate layer 301 includes the first surface304 and the bulk layer 302 includes the second surface 306. The bulklayer 302 disposed over the intermediate layer 301. The thickness 308includes a thickness 303 of the intermediate layer 301 and thickness 305of the bulk layer 302. The intermediate layer 301 has a thickness 303that is not greater than about 20% of the thickness 308. Theintermediate layer 301 has a density not greater than the density of thebulk layer 302. In one embodiment, the density of the bulk layer 302 isgreater than about 2.15 g/cm³. ⁻²⁻²Referring to FIG. 2, the firstsurface 304 of the gate dielectric layer 208B is in contact with thesemiconductor layer 206 at the interface 201. The gate dielectric layer208B has the D_(it) of about 5e¹⁰ cm⁻² eV⁻¹ to about 5e¹¹ cm⁻²eV⁻¹ andthe hysteresis of about 0.10 V to about 0.30 V.

FIG. 4 is a flow diagram of a method 400 of fabricating a top gate TFT.In one embodiment, the chamber 100 is utilized for the method 400. Atoptional operation 401, initial layers of the TFT are formed on asubstrate 202. In one embodiment, a buffer layer 204 is deposited overthe substrate 202 and the semiconductor layer 206 is deposited over thebuffer layer 204 by PECVD processing. At operation 402, the gatedielectric layer 208 is deposited.

In one embodiment, a gate dielectric layer 208A is deposited by asingle-step PECVD process. The single-step PECVD process includesflowing a deposition gas at a deposition gas flow rate into the processvolume 106 of the chamber 100 and applying RF power to the depositiongas. In one embodiment, the deposition gas includes silane (SiH₄),nitrous oxide (N₂O), and Argon (Ar). The deposition gas flow rateincludes about 800 sccm to about 1600 sccm of SiH₄, about 30000 sccm toabout 70000 sccm of N₂O, and about 40000 sccm to about 70000 sccm of Ar.In one embodiment, the RF power source 122 provides RF power to thediffuser 110. The RF power is applied an initial power level and afrequency at an initial interval. The initial power level is increasedin intervals, also known as ramped, until the RF power is applied at afinal power level at a final interval. In one embodiment, the intervalsare 0.5 about seconds to about 2 seconds. The initial power level may beincrease in a step-wise, exponentially, or literally in the intervalsuntil the RF power is applied at a final power level. In one embodiment,the RF power is about 2000 Watts (W) to about 16000 W. For example, theinitial power level is about 2000 W to about 5000 W and the final powerlevel is about 12000 W to about 16000 W. Increasing the initial powerresults in the gate dielectric layer 208A divided into a range of zones310 measured from the first surface 304 corresponding to 0% of thethickness 308 to the second surface 306 corresponding to 100% of thethickness 308. Each zone 312 has a zone thickness and zone density. Inone embodiment, each zone 312 has a zone thickness of about 0.001% toabout 20% of the thickness 308.

A density profile through the range of zones 310 of the thickness 308has a minimum density and a maximum density. In one embodiment, aninitial zone of the range of zones 310 adjacent to the first surface 304and corresponding to the initial interval has a zone density with theminimum density, the final zone 316 of the range of zones 310 adjacentto the second surface 306 and corresponding to the final interval has azone density with the maximum density, and the zone density each zone312 deposited immediately over a prior zone is not less than the zonedensity of the prior zone. The density profile is a result of increasingthe initial power level in intervals that reduces plasma damage to anunderlying layer from ion bombardment. In one embodiment, each zone 312has a zone thickness that is substantially the same and each zone 312disposed immediately over a prior zone has a zone density not less thanthe zone density of the prior zone. In another embodiment, the increasein the zone density is a result of ramping the RF power. Applying the RFpower to the deposition gas for the initial interval at the initialpower level forming the initial zone 314 a range of zones 310,increasing the initial power level in intervals forming zones 312 of therange of zones 310 until the RF power is applied for a final interval ata final power level forming a final zone 316 of the range of zones mayresult in each zone 312 of the range of zones 310 having substantiallythe same thickness and each zone 312 disposed immediately over a priorzone having a zone density not less than the zone density of the priorzone.

In another embodiment, an initial pressure in the process volume 106 atthe initial interval is decreased in the intervals until a finalpressure at the final interval. In one embodiment, the initial pressureof about 900 mTorr to about 1300 mTorr is decreased in the intervals.Each zone 312 of the range of zones 310 corresponding to each intervalhas a zone thickness and a zone density. The initial zone of the rangeof zones 310 corresponding to the initial interval has a zone densitywith the minimum density, the final zone 316 of the range of zones 310corresponding to the final interval has a zone density with the maximumdensity, and the zone density each zone deposited immediately over aprior zone has a zone density not less than the zone density of theprior zone.

In one embodiment, a gate dielectric layer 208B is deposited by atwo-step PECVD process. The two-step PECVD process includes flowing adeposition gas at a deposition gas flow rate into the process volume 106of the chamber 100 and applying RF power to the deposition gas. In oneembodiment, the RF power source 122 provides RF power to the diffuser110. The RF power is applied an initial power level and a frequency andthe process volume 106 has an initial pressure for an initial intervaluntil an intermediate layer 301 of the gate dielectric layer 208B isdeposited. The RF power is applied a final power level and the processvolume 106 has a final pressure for a final interval until a bulk layer302 of the gate dielectric layer 208B is deposited. In one embodiment,the initial power level is greater than the final power level. Inanother embodiment, the final pressure is less than the initialpressure. In one embodiment, the RF power is about 2000 Watts (W) toabout 16000 W. For example, the initial power level is about 2000 W toabout 5000 W and the final power level is about 12000 W to about 16000W. In one embodiment, the initial pressure is about 900 mTorr to about1300 mTorr. The intermediate layer 301 has a thickness 303 that is notgreater than about 20% of the thickness 308. The intermediate layer 301has a density not greater than the density of the bulk layer 302. Theintermediate layer 301 deposited at the initial power level reducesplasma damage to an underlying layer from ion bombardment at the finalpower level depositing the bulk layer 302 of the gate dielectric layer208B.

At operation 403, the remaining structure of the TFT is formed. In oneembodiment, a gate electrode 210 is formed over the gate dielectriclayer 208 by PVD processing. An ILD layer 212 is deposited over the gateelectrode 210 by PECVD processing. A source electrode 214 is formed byPVD processing in a source electrode via 218 formed in the ILD layer 212and a drain electrode 216 is formed by PVD processing in a drainelectrode via 220 formed in the ILD layer 212.

In summation, TFTs and methods of fabricating TFTs having a gatedielectric layer are described herein. The gate dielectric layer has aD_(it) of about 5e¹⁰ cm⁻²eV⁻¹ to about 5e¹¹ cm⁻²eV⁻¹ and a hysteresis ofabout 0.10 V to about 0.30 V to improve performance capability of thetop gate TFT 200 while having a breakdown field between about 6 MV/cmand about 10 MV/cm. The single-step PECVD process forms a gatedielectric layer with a density profile through range of zones of thethickness of the gate dielectric layer. The initial zone of the range ofzones adjacent to the underlying layer has a zone density with theminimum density, the finial zone of the range of has a zone density withthe maximum density, and the zone density each zone depositedimmediately over a prior zone is not less than the zone density of theprior zone. The density profile is a result of increasing the initialpower level in intervals that reduces plasma damage to an underlyinglayer from ion bombardment. The two-step PECVD process forms a gatedielectric layer with an intermediate layer and a bulk layer. Theintermediate layer has a thickness that is not greater than about 20% ofthe thickness of the gate dielectric layer and has a density not greaterthan the density of the bulk layer. The intermediate layer deposited atthe initial power level reduces plasma damage to an underlying layerfrom ion bombardment at the final power level depositing the bulk layerof the gate dielectric layer.

While the foregoing is directed to examples of the present disclosure,other and further examples of the disclosure may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

What is claimed is:
 1. A thin film transistor (TFT), comprising: asubstrate; a buffer layer disposed on the substrate; a semiconductorlayer disposed on the buffer layer; a gate dielectric layer disposed onthe semiconductor layer, the gate dielectric layer contacting thesemiconductor layer at an interface, the gate dielectric layer havingbreakdown field between about 6 MV/cm and about 10 MV/cm, an interfacetrap density (D_(it)) of about 5e¹⁰ cm⁻² eV⁻¹ to about 5e¹¹ cm⁻² eV⁻¹,and a hysteresis of about 0.10 V to about 0.30 V; a gate electrodedisposed on the gate dielectric layer; an inter-layer dielectric (ILD)layer disposed on the gate dielectric layer and the gate electrode; anda source electrode disposed in a source electrode via of the ILD layerand a drain electrode is disposed in a drain electrode via of the ILDlayer.
 2. The TFT of claim 1, wherein the gate dielectric layercomprises: a first surface contacting the semiconductor layer at theinterface; a second surface contacting the gate electrode and the ILDlayer; a thickness divided into a range of zones from the first surfacecorresponding to 0% of the thickness to the second surface correspondingto 100% of the thickness, each zone of the range of zones having a zonethickness and zone density; and a density profile through the range ofzones having a minimum density and a maximum density, wherein: aninitial zone of the range of zones adjacent to the first surface has thezone density with the minimum density; and a finial zone of the range ofzones adjacent to the second surface has the zone density with themaximum density, the zone density each zone disposed immediately over aprior zone is not less than the zone density of the prior zone.
 3. TheTFT of claim 2, wherein the zone thickness is about 0.001% to about 20%of the thickness.
 4. The TFT of claim 1, wherein the gate dielectriclayer comprises: an intermediate layer having a first surface contactingthe semiconductor layer at the interface with the first surface; a bulklayer having a second surface contacting the gate electrode and the ILDlayer; and a thickness consisting essentially of the intermediate layerand the bulk layer, the intermediate layer is not greater than about 20%of the thickness and has a density not greater than the density of thebulk layer.
 5. The TFT of claim 1, wherein the gate dielectric layer hasa thickness of about 500 Angstroms to about 1500 Angstroms.
 6. The TFTof claim 1, wherein the buffer layer comprises at least one of a siliconmononitride (SiN) and a silicon dioxide (SiO₂) containing material, orthe SiO₂ containing material.
 7. The TFT of claim 1, wherein thesemiconductor layer includes at least one of a silicon, a polysilicon, alow temperature polysilicon, an amorphous silicon, anindium-gallium-zinc oxide (IGZO), and an zinc oxynitride (ZnON)containing material.
 8. The TFT of claim 1, wherein the gate dielectriclayer comprises at least one of a SiN, a silicon nitride (Si₃N₄), asilicon monoxide (SiO), a silicon dioxide SiO₂, and a silicon oxynitrideSi₂N₂O containing material.
 9. The TFT of claim 1, wherein the gateelectrode, the source electrode, and the drain electrode each comprisesat least one of a copper, a titanium, and a tantalum containingmaterial.
 10. A thin film transistor (TFT), comprising: a substrate; abuffer layer disposed on the substrate; a semiconductor layer disposedon the buffer layer; a gate dielectric layer disposed on thesemiconductor layer, the gate dielectric layer having a first surfaceand a second surface, the first surface contacts the semiconductor layerat an interface, the gate dielectric layer has a breakdown field betweenabout 6 MV/cm and about 10 MV/cm, an interface trap density (DO of about5e¹⁰ cm⁻²eV⁻¹ to about 5e¹¹ cm⁻²eV⁻¹, and a hysteresis of about 0.10 Vto about 0.30 V, the gate dielectric layer comprises: a thicknessdivided into a range of zones from the first surface corresponding to 0%of the thickness to the second surface corresponding to 100% of thethickness, each zone of the range of zones having a zone thickness andzone density; and a density profile through the range of zones having aminimum density and a maximum density, wherein: an initial zone of therange of zones adjacent to the first surface has the zone density withthe minimum density; and a finial zone of the range of zones adjacent tothe second surface has the zone density with the maximum density, thezone density each zone disposed immediately over a prior zone is notless than the zone density of the prior zone; a gate electrode disposedon the gate dielectric layer, the gate electrode contacting the secondsurface of the gate dielectric layer, an inter-layer dielectric (ILD)layer disposed on the gate dielectric layer and the gate electrode; anda source electrode disposed in a source electrode via of the ILD layerand a drain electrode is disposed in a drain electrode via of the ILDlayer.
 11. The TFT of claim 10, wherein the zone thickness is about0.001% to about 20% of the thickness.
 12. The TFT of claim 10, whereinthe gate dielectric layer has a thickness of about 500 Angstroms toabout 1500 Angstroms.
 13. A method of fabricating a thin film transistor(TFT), comprising: flowing a deposition gas at a deposition gas flowrate into a process volume of a chamber; applying a radio frequency (RF)power to the deposition gas for an initial interval at an initial powerlevel forming an initial zone a range of zones of a gate dielectriclayer, the initial zone having a zone density with a minimum density;and increasing the initial power level in intervals forming zones of therange of zones until the RF power is applied for a final interval at afinal power level forming a final zone of the range of zones, the finalzone having the zone density with a maximum density, and the zonedensity of each zone formed has a density not less than the zone densityof a prior zone.
 14. The method of claim 13, further comprising forminginitial layers of the TFT on a substrate before flowing the depositiongas at the deposition gas flow rate.
 15. The method of claim 13, furthercomprising forming a remaining structure of the TFT after the RF poweris applied for the final interval at the final power level.
 16. Themethod of claim 13, wherein the deposition gas is flowed at an initialpressure at the initial interval and the initial pressure is decreasedin the intervals until a final pressure at the final interval.
 17. Themethod of claim 13, wherein the initial power level is increasedstep-wise, exponentially, or linearly in the intervals until the RFpower is applied at the final power level.
 18. The method of claim 13,wherein the RF power is about 2000 Watts (W) to about 16000 W.
 19. Themethod of claim 13, wherein the initial power level is about 2000 W toabout 5000 W.
 20. The method of claim 13, wherein the final power levelis about 12000 W to about 16000 W.